The present invention relates generally to endodontic instruments, such as files and reamers and, more specifically, to those instruments especially useful in root canal procedures.
Endodontists use various types of instruments for cleaning and enlarging the root canals of the teeth. In a typical root canal procedure, an endodontist first makes an opening in the surface of the tooth to provide access to the interior. The endodontist then utilizes small instruments, such as hand held files and reamers, to clean and enlarge the narrow, tapered root canals. In a conventional procedure, the endodontist fills the prepared root canals with gutta percha, which is a rubber-like substance, and then seals the tooth with protective cement. The endodontists may sometimes apply a crown to the tooth as a final step.
Typically, the endodontist uses a series of delicate, flexible files to clean out and shape the root canals. Each file includes a proximal end which includes a handle to be gripped between the fingers of the endodontist. The files each further include a distal end or tip. A working length with helical flutes and cutting edges is located between the proximal and distal ends. The endodontist uses files of increasingly larger diameter to sequentially increase the diameter of the root canal and achieve the desired diameter and shape.
Endodontic root canal files and reamers have been formed from twisted blanks in generally three different configurations. One type is formed by twisting a ground blank having a square cross section to create four helical cutting edges per revolution. Another type consists of a twisted blank of triangular cross section having three cutting edges per revolution. The third type, often referred to as a K-flex type, is formed from a blank having a parallelogram-shaped cross section, such as a rhomboid-shaped cross section. After twisting this type of blank, two cutting edges and two debris removal edges will be formed per revolution. All three of these types of instruments have a tapered major diameter or cross-sectional dimension and a tapered minor diameter or cross-sectional dimension in which the taper angles are generally the same. Also, the angles formed between the surfaces that define the cutting and debris removal edges are constant along the length of the instrument. In other words, a given grind angle of an edge on the instrument remains the same along the entire working length of that instrument.
Existing endodontic files and reamers formed from twisted blanks are designed in such a manner that the minor diameter is purely a function of the major diameter. The undesirable consequences of this type of design become significant for instruments that have a greater taper along the working length. In particular, these instruments become much stiffer toward the proximal end or handle of the instrument. This can cause the instrument to be difficult to maneuver within curved root canals because the instrument may not flex enough to conform to the shape of the canal. Although certain helically fluted endodontic instruments have been formed completely by grinding to achieve more constant flexibility along the length, these instruments have significant drawbacks. First, instruments formed completely by grinding are more costly to manufacture. Also, twisted instruments may be formed in a wide variety of cross-sectional shapes, depending on the shape of a initially ground wire blank.
In view of problems in this field, including those problems noted above, it would be desirable to provide an endodontic instrument, such as a file or reamer formed from a twisted blank, in which the size of one diameter or cross-sectional dimension is formed independent of the other to optimize flexibility, strength and other operating characteristics of the instrument. In this manner, instruments of greater taper may be formed with greater flexibility for maneuvering within curved root canals, while also retaining sufficient strength to resist breakage during use.
One improved instrument was disclosed in U.S. patent application Ser. No. 09/288,173, filed Apr. 8, 1999, now pending, assigned to the assignee of the present invention. The disclosure of this prior related application is hereby fully incorporated by reference herein. The instruments disclosed in this prior application utilize a blank in which the grind angles of the edges vary along the length of the blank. This advantageously allows for more flexibility of the instrument, especially in those instruments having a greater taper with increased dimensions at the proximal end of the working length. Nevertheless, there is still a need for improvements in relation to solving the same type of flexibility problems while reducing the difficulty and expense associated with manufacturing the instruments.
The present invention provides an endodontic instrument, such as a file, reamer or other cutting, shaping or cleaning instrument, comprising an elongate member with optimal flexibility, strength and other operating characteristics. The elongate member includes a longitudinal axis, which is preferably straight when not in use, and a proximal end, a distal end and a working length generally between the proximal and distal ends. The working length is formed with an outer surface comprising a plurality of lengthwise extending surface portions ground therefrom. A plurality of lengthwise extending edges are respectively positioned between adjacent lengthwise extending surface portions. In accordance with one aspect of this invention, the minor diameter is formed independent of the major diameter at one or more locations along the working length. In this manner, the minor diameter may be of any dimension below the major diameter depending on the desired characteristics. As an illustrative example, this may be accomplished by removing at least a portion of one of the lengthwise extending edges of the elongate member to form another surface portion along which the flexibility of the instrument is increased and which defines the minor diameter of the instrument at the location of material removal.
In the preferred embodiments, the principles of this invention are applied to instruments having generally triangular cross sections or generally parallelogram-shaped cross sections. The working length of the instrument is preferably tapered in a direction from the proximal end toward the distal end such that the diameter of the working length decreases in that direction. One or more cutting edges are located along the major diameter of the working length and, in the preferred embodiments, opposite edges lying on the minor diameter are ground off as additional lengthwise extending surface portions that preferably taper less than the remaining lengthwise extending surface portions which define cutting edges therebetween. More preferably, the additional surface portions are formed as zero taper flats.
In accordance with the invention, the instrument will be more flexible than conventional instruments of the same taper. As mentioned above, the invention allows the minor and major diameters of the instrument to be sized independent of one another. This aspect allows the minor diameter or cross sectional dimension to be maintained substantially constant along the working length, while the major diameter or cross sectional dimension includes a taper. Thus, a twisted instrument according to this aspect of the invention will have a more constant flexibility along the working length notwithstanding a significant taper existing along the major diameter. Other embodiments of this general aspect are also possible and include, for example, forming the minor and major diameters with different tapers.
Endodontic instruments of this invention may be formed with many different cross sectional shapes. Typically, the elongate member of the finished instrument will have three or four longitudinally or lengthwise extending helical surface portions and at least one longitudinal, helically-shaped cutting edge. The instrument may be formed from materials having superelastic properties and/or other materials, such as titanium, carbon steel or stainless steel.
A preferred method of making endodontic instruments according to the invention includes removing material from an outer surface of a wire along at least three separate paths extending along the working length to form adjacent first, second and third lengthwise surface portions. The method further includes forming at least three separate edges extending along the working length and each positioned between two adjacent lengthwise surface portions. At least a portion of one of the edges is removed to form at least a fourth lengthwise surface portion and to decrease the diameter of the blank along the fourth lengthwise surface portion. The formed blank is then twisted to form the edges and lengthwise surface portions into helically-shaped edges and surface portions along the working length.
Other features, objects and advantages of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings.